The invention relates generally to the field of drill pipe used in drilling wellbores through subsurface formations. More particularly, the invention relates to “dual” drill pipe, in which two separate fluid conduits are provided within a single pipe extending from a drilling unit into a wellbore.
Dual drill pipe is disclosed, for example, in U.S. Pat. No. 3,208,539 issued to Henderson. Generally, dual drill pipe includes conventional, threadedly connected drill pipe, such as conforms to standards set by the American Petroleum Institute, Washington, D.C. An inner conduit or tube may be disposed in the interior of the conventional drill pipe to provide an additional fluid conduit. As explained in the Henderson '539 patent, the additional fluid conduit may be used to transport drill cuttings from a drill bit back to the surface during drilling operations. A possible advantage of using the additional conduit for such purpose is that the conduit has smaller cross-sectional area than an annular space between the exterior of the conventional drill pipe and the wall of the wellbore. Such smaller cross-sectional area enables higher drilling fluid velocity, thus requiring less drilling fluid flow to entrain the drill cuttings and return them to the surface. Another possible advantage of using the additional conduit for cuttings return is in drilling highly inclined wellbores. In such wellbores, cuttings tend to settle on the bottom of the wellbore, sometimes leading to “packing”, and having the drill pipe become stuck in the wellbore as a result.
Dual drill pipe structures known in the art may require extended seal engagement features for the inner tube to be sealingly engaged between segments of the dual drill pipe. Dual dill pipe structures may also require expensive and difficult manufacturing techniques. There exists a need for improved structures for dual drill pipe.